The invention relates to a vehicle yaw dynamics control system, capable of optimally controlling a vehicle yaw momentum during steering operation by applying a yawing moment to the vehicle by means of a control apparatus which performs a driving-force distribution control and/or a braking-force control for each of road wheels.
A conventional vehicle yaw dynamics control system has been disclosed in Japanese Patent Provisional Publication No. 5-262156.
The conventional system includes a driving-force distribution adjusting mechanism which distributes a driving force output from an engine of an automotive vehicle into left and right road wheels and adjusts torque-distribution between the left and right road wheels, a yaw rate sensor which detects an actual yaw rate of the automotive vehicle, a target yaw rate arithmetic-calculation means which arithmetically calculates a target yaw rate on the basis of input information, namely a steer-angle information data signal from a steer angle sensor and a vehicle-speed information data signal from a vehicle speed sensor, and a control means which controls the operation of the driving-force distribution adjusting mechanism. The control means is constructed in a manner so as to set a controlled hydraulic pressure applied to the driving-force distribution adjusting mechanism, while performing feed-back control so that the actual yaw rate is approached to the target yaw rate.
The previously-described conventional system has feedback-controlled so that the actual yaw rate is approached or adjusted to the target yaw rate. However, it is a somewhat delay time from the occurrence of a yaw moment applied to the vehicle after operation of the driving-force distribution adjusting mechanism to a time when the yaw moment generated is detected as a yaw rate by means of the yaw rate sensor. Thus, in performing the feedback control based on the actual yaw rate detected in a way as discussed above, as shown in FIG. 22(b), there is a risk or possibility of control delay.
Additionally, a differential calculus is made to obtain a controlled variable, thus resulting in increased oscillations and noises. In cooperation with the previously-noted control delay, as a result of the yaw dynamics control, the characteristic curve tends to oscillate, as shown in FIG. 23.
There is the problem that the driver feels uncomfortable in the presence of the above-mentioned undesired control delay or oscillations (hunting).
In addition, a yaw moment, acting on the vehicle, is dependent on a side force acting at a tire. The side force varies depending on a friction coefficient of a road surface (which will be hereinafter referred to as a xe2x80x9croad-surface xcexcxe2x80x9d). The prior-art system could not execute a high-precision yaw control in due consideration of the road-surface xcexc.
It is, therefore, in view of the previously-described disadvantages of the prior art, in controlling a yaw momentum acting on the vehicle, a principal object of the present invention to provide a vehicle yaw dynamics control system which is capable of enhancing the quality of yaw control without giving a driver an uncomfortable feeling, while eliminating the problem of the control delay or the occurrence of hunting during the yaw control. Furthermore, it is another object of the invention is to attain the previously-noted principal object, in spite of a low-cost yaw dynamics control system. It is a still further object of the invention is to highly enhance the quality of yaw control by performing the yaw control depending on the road-surface xcexc.
In order to accomplish the aforementioned objects, as shown for example in the block diagram of FIG. 1, a vehicle yaw dynamics control system of the present invention, a yaw moment generating mechanism a which produces a yawing motion at the vehicle, a vehicle behavior detection means b which detects a vehicle behavior, an actual yaw moment detection means c which is included in the vehicle behavior detection means b and detects an actual yaw moment acting on the vehicle, a target yaw moment arithmetic-calculation means d which arithmetically calculates a target yaw moment necessary for a current vehicle behavior on the basis of the latest up-to-date input information being input from the vehicle behavior detection means b, and an operating command means e which operates the yaw moment generating mechanism to output a yaw moment equivalent to the difference between the target yaw moment and the actual yaw moment.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted vehicle behavior detection means b includes a side-force plus longitudinal-force detection means which detects a side force acting on each of road wheels and a longitudinal force acting on each of the road wheels, and the previously-noted actual yaw moment detection means a includes a means for arithmetically calculating the actual yaw moment on the basis of an input from the side-force plus longitudinal-force detection means.
According to aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted vehicle behavior detection means b includes a yaw rate sensor which detects a yaw rate of the vehicle, and the previously-noted actual yaw moment detection means c includes a means for arithmetically calculating the actual yaw moment by multiplying a differentiated value of the yaw rate detected with a yaw moment of inertia of the vehicle.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted target yaw moment arithmetic-calculation means d includes a means for arithmetically calculating a target yaw rate by a steer angle and a quantity of state of the vehicle, and for arithmetically calculating the target yaw moment by multiplying the a differentiated value of the target yaw rate with a yaw moment of inertia of the vehicle.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted target yaw moment arithmetic-calculation means d includes a means for arithmetically calculating the target yaw moment by a quantity of state of each of the road wheels and a target tire characteristic.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted target yaw moment arithmetic-calculation means d includes a wheel load arithmetic-calculation means d1 which arithmetically calculates a wheel load of each of the road wheels, a wheel slip angle arithmetic-calculation means d2 which arithmetically calculates a slip angle of each of the road wheels, and a wheel braking-force/driving-force arithmetic-calculation means d3 which arithmetically calculates a wheel braking-force/driving-force, and the quantity of state of each of the road wheels comprises a wheel load, a slip angle, and a braking-force/driving-force.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted target yaw moment arithmetic-calculation means d includes a load-transfer arithmetic-calculation means d11 for arithmetically calculating a load transfer based on lateral acceleration, a slip-angle arithmetic-calculation means d12 for arithmetically calculating a slip angle of each of the road wheels, and an arithmetic-calculation means d13 for arithmetically calculating a target side force by only the load transfer and the slip angle of each of the road wheels, from the target tire characteristic, and for arithmetically calculating the target yaw moment on the basis of the target side force.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted actual yaw moment detection means c is constructed by a tire quantity-of-state estimation means c1 for estimating a quantity of state of each of tires of the road wheels, and an arithmetic-calculation means c2 for arithmetically calculating the yaw moment of the vehicle by an output signal from the tire quantity-of-state estimation means c1.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted vehicle behavior detection means b includes a lateral acceleration sensor, a longitudinal acceleration sensor, a brake sensor, a steer angle sensor, a yaw rate sensor, a vehicle speed sensor, and a vehicle slip angle detection means, and the tire quantity-of-state estimation means c1 for each of tires of the road wheels includes a wheel slip angle arithmetic-calculation means all for arithmetically calculating a slip angle of each of the road wheels on the basis of a vehicle slip angle, a steer angle, a yaw rate, and a vehicle speed, a wheel load arithmetic-calculation means c12 for arithmetically calculating a wheel load of each of the road wheels on the basis of a longitudinal acceleration acting in the longitudinal direction of the vehicle and a lateral acceleration acting in the lateral direction of the vehicle, a braking-force/driving-force arithmetic-calculation means c13 for arithmetically calculating a braking-force/driving-force acting on each of the road wheels on the basis of a braking condition and the longitudinal acceleration acting in the longitudinal direction of the vehicle, and a side force arithmetic-calculation means c14 for arithmetically calculating a side force on the basis of the wheel load of each of the road wheels, the braking-force/driving-force acting on each of the road wheels, and the slip angle of each of the road wheels, which data are obtained by way of these arithmetic-calculation means c11, c12, and c13.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted side force arithmetic-calculation means c14 includes a means for retrieving a side force acting on each of the road wheels on the basis of the wheel load and the slip angle except the braking-force/driving-force, from a preset characteristic map, a means for arithmetically calculating a side-force reduction rate on the basis of the braking-force/driving-force, and a means for arithmetically calculating the side force of each of the road wheels on the basis of the side force retrieved and the side-force reduction rate except the braking-force/driving-force.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted wheel slip angle arithmetic-calculation means d2 is constructed so that the slip angle of each of the road wheels is arithmetically calculated after a slip angle of the center of gravity of the vehicle is calculated in accordance with the following procedures. In calculating the slip angle B of the center of gravity of the vehicle, the wheel slip angle arithmetic-calculation means includes an arithmetic-calculation means for arithmetically calculating, first of all, a cornering-power estimate PC2 of a rear wheel of the road wheels on the basis of signals from the respective sensors, namely a yaw-rate indicative signal xcex94"psgr", a lateral acceleration indicative signal xcex94xcex94Y, and a vehicle speed indicative signal V, from the following expression (1).
PC2=(V/L)(maxcex94xcex94Yxe2x88x92Ixcex94"psgr"s)s/[xcex94"psgr"(bs+V)xe2x88x92xcex94xcex94Y]+f(xcex94xcex94Y)xe2x80x83xe2x80x83(1)
where s denotes a Laplace operator, m denotes a mass of the vehicle, a denotes a longitudinal distance from the center of gravity of the vehicle to a front-wheel axle, b denotes a longitudinal distance from the center of gravity of the vehicle to a rear-wheel axle, L denotes a wheel base, I denotes a moment of inertia of the vehicle, a first term of the right side of the expression is a rear-wheel cornering power calculated analytically from a two-wheel model of the vehicle, and a second term f(xcex94xcex94Y) is a correction term based on the lateral acceleration. Second, the arithmetic-calculation means arithmetically calculates a slip angle xcex2 by the rear-wheel cornering power estimate PC2 and the yaw rate indicative signal xcex94"psgr", from the following expression (2) representative of the relationship between the yaw rate calculated analytically from a two-wheel model of the vehicle, and the slip angle.
xcex2=xe2x88x92Kbr[(Tbs+1)/(Trs+1)]xcex94"psgr"xe2x80x83xe2x80x83(2)
where Kbr=(1xe2x88x92(ma/(LbPC2))V2)(b/V), Tb=IV/(LbPC2xe2x88x92maV2), and Tr=[ma/(LPC2)]V.
Alternatively, according to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted wheel slip angle arithmetic-calculation means d2 is constructed so that the slip angle of each of the road wheels is arithmetically calculated after a slip angle of the center of gravity of the vehicle is calculated as follows. In calculating the slip angle of the center of gravity of the vehicle, the wheel slip angle arithmetic-calculation means includes an arithmetic-calculation means for arithmetically calculating, first of all, a cornering-power estimate PC2 of a rear wheel of the road wheels on the basis of signals from the respective sensors, namely a yaw-rate indicative signal xcex94"psgr", a lateral acceleration indicative signal xcex94xcex94Y, and a vehicle speed indicative signal V, from the following expression (5).
PC2=(V/C)(maxcex94xcex94Yxe2x88x92Ixcex94"psgr"s)s/[xcex94"psgr"(bs+V)xe2x88x92xcex94xcex94Y]xe2x80x83xe2x80x83(5)
where s denotes a Laplace operator, m denotes a mass of the vehicle, a denotes a longitudinal distance from the center of gravity of the vehicle to a front-wheel axle, b denotes a longitudinal distance from the center of gravity of the vehicle to a rear-wheel axle, L denotes a wheel base, and I denotes a moment of inertia of the vehicle. Second, the arithmetic-calculation means arithmetically calculates a slip angle xcex2 by the rear-wheel cornering power estimate PC2 and the yaw rate indicative signal xcex94"psgr", from the following expression (6) representative of the relationship between the yaw rate calculated analytically from a two-wheel model of the vehicle, and the slip angle.
xcex2=xe2x88x92Kbr[(Tbs+1)/(Trs+1)]xcex94"psgr"xe2x80x83xe2x80x83(6)
where Kbr=(1xe2x88x92(ma/(LbPC2))V2)(b/V), Tb=IV/(LbPC2xe2x88x92maV2), and Tr=[ma/(LPC2)]V.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted target yaw moment arithmetic-calculation means d and the previously-noted actual yaw moment arithmetic-calculation means c respectively calculates the target side force and the actual side force, on the basis of the quantity of state of each of the road wheels and tire characteristics written as an arithmetic expression, and then arithmetically calculates the target yaw moment from the target side force and also calculates the actual yaw moment from the actual side force.
According to another aspect of the invention, in the vehicle yaw dynamics control system, an arithmetic expression for arithmetic-calculation of the actual side force Fsi performed by means of the previously-noted actual yaw moment arithmetic-calculation means c is as follows.
Fsi=Limit[xcex3ixe2x88x92(xcex3i2/3)+(xcex3i3/27)][(xcexcWi)2xe2x88x92Fai2]xc2xd
Additionally, in the previously-noted target yaw moment arithmetic-calculation means d, an arithmetic expression for arithmetic-calculation of the target side force F"Asteriskpseud"si is as follows.
F"Asteriskpseud"si={Limit[xcex3"Asteriskpseud"ixe2x88x92(xcex3"Asteriskpseud"i2/3)+(xcex3"Asteriskpseud"i3/27)]+Axcex2i}[(xcexcWi)2xe2x88x92BFai2]xc2xd
where the previously-noted xcex3i means xcex3i=|(Kc/xcexcWi) tan xcex2i|, and the function Limit[xcex3ixe2x88x92(xcex3i2/3)+(xcex3i3/27)] is a characteristic function that is saturated when a value within [ ] exceeds xe2x80x9c1xe2x80x9d, and the above-mentioned character Wi denotes a wheel load of each road wheel, the above-mentioned character xcex2i denotes a side slip angle of each road wheel, the above-mentioned character Fai denotes a braking-force/driving-force, the above-mentioned character Kc denotes a cornering stiffness, the above-mentioned character xcexc denotes a friction coefficient between the tire and the road surface, the character A denotes a constant, and the character B denotes a longitudinal force correction factor. Also, xcex3"Asteriskpseud"i means
xcex3"Asteriskpseud"i=|(K"Asteriskpseud"c/xcexcWi) tan xcex2i|, 
and K"Asteriskpseud"c denotes a cornering stiffness.
According to another aspect of the invention, in the vehicle yaw dynamics control system, an arithmetic expression for arithmetic-calculation of the actual side force Fsi performed by means of the previously-noted actual yaw moment arithmetic-calculation means c is as follows. Fsi=D sin {C arctan[Fxe2x88x92E(Fxe2x88x92arctan(F))])} where each of C, D, E, and F is a function of xcexc, Wi, and xcex2i.
Additionally, an arithmetic expression for arithmetic-calculation of the target side force F"Asteriskpseud"si performed by means of the previously-noted target yaw moment arithmetic-calculation means d is as follows.
F"Asteriskpseud"Si=D"Asteriskpseud"{sin [C"Asteriskpseud"arctan[F"Asteriskpseud"xe2x88x92E"Asteriskpseud"(F"Asteriskpseud"xe2x88x92arctan(F"Asteriskpseud")))]+Z xcex2i}
where each of C"Asteriskpseud", D"Asteriskpseud", E"Asteriskpseud", and F"Asteriskpseud" is a function of xcexc, Wi, and xcex2i, and Z is a constant.
According to another aspect of the invention, in the vehicle yaw dynamics control system, a road-surface friction coefficient detection means is provided for detecting a road-surface friction coefficient. The previously-noted actual yaw moment arithmetic-calculation means is constructed so that arithmetic-calculation for the actual yaw moment is varied depending on the road surface friction coefficient.
According to another aspect of the invention, in the vehicle yaw dynamics control system, a road-surface friction coefficient detection means is provided for detecting a road-surface friction coefficient. The previously-noted target yaw moment arithmetic-calculation means is constructed so that arithmetic-calculation for the target yaw moment is varied depending on the road surface friction coefficient.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the previously-noted road surface friction coefficient detection means is constructed so that the road surface friction coefficient is estimated by a ratio of the longitudinal acceleration of the vehicle to the slip rate of a drive wheel.
According to another aspect of the invention, in the vehicle yaw dynamics control system, the road surface friction coefficient detection means is constructed so that the previously-noted road surface friction coefficient xcexc is derived from the following arithmetic expression, on the assumption that the previously-noted slip rate of the drive wheel is denoted by s, the tire stiffness is denoted by K, the driving force F is defined by an equation F=Ks, the longitudinal acceleration is denoted by Ax, and the vehicle weight is denoted by m.
xcexc=(mAx/F)=(mAx/Ks)
According the system of the invention, during vehicle driving, the target yaw moment arithmetic-calculation means d arithmetically calculates a target yaw moment necessary for a current vehicle behavior, while the actual yaw moment detection means c detects an actual yaw moment actually acting on the vehicle. Additionally, the operating command means e operates the yaw moment generating mechanism a to output a yaw moment equivalent to the difference between the target yaw moment and the actual yaw moment.
In this manner, according to the invention, a desired value or target value and a detected value are both are derived as yaw moments, and then these values are compared to each other for yaw dynamics control, and whereby there is less delay in control results and oscillations (hunting).